Fuel assemblies very frequently used comprise a bundle of fuel elements (each formed by a stack of pellets enclosed in a sealed sheath) held in position in a structure. This latter comprises grids spaced apart along the assembly and formed by plates disposed in two orthogonal directions, for defining cells through the major part of which elements pass. Each of these cells is provided with means bearing on the element which passes through the cell for supporting it.
These means may be formed by a part punched out from the plates forming lugs bearing against the elements. But it is more advantageous to use springs fixed to the plates, which may be made from a material different from the plates. A grid of this latter type is described, for example, in French Patent Application No. 2,474,229, to which reference may be made.
The springs in general have a hair pin shape with two legs. Generally, several types of spring are provided in the same grid because the grid has edges, and often some cells are occupied by elements other than fuel elements, for example by guide tubes. One of the types will frequently comprise two identical legs, whereas another type will have different legs. These springs are first of all inserted, then finally secured to the plates by welding the two legs to each other at well defined points, between which are located windows formed in the grid plate.
The positioning of each spring is a delicate operation: it is necessary to locate them accurately so as not to squeeze them on the plates during the welding operation, and so as to avoid any contamination. To date, the springs have been fitted and welded manually, which is a long and difficult operation, likely moreover to give rise to errors when springs of different types are to be placed in well defined zones of the plate. It is possible that one of the reasons for which this process has not been automated resides in the risk of the legs of the spring catching on the numerous projections of the plate during insertion of the spring whose legs are spaced apart by a distance which cannot be exactly defined because of their elasticity.
French Pat. No. 82 17717 further describes a process and device for positioning and welding hair pin springs. During this process, the plate is moved step by step so as to bring all the zones of the plate to be provided with springs successively into an insertion position where a spring is slid into place by moving along an orientation close to the plane of the plate, in a direction perpendicular to the length of the plate. So as to prevent the springs from catching in the windows, these latter must be covered with a mask during insertion. The mask is then removed for spot-welding of the legs of the spring through the windows.
Although this process has the advantage of being readily automatable, it has the drawback of being relatively slow, because of the time required for positioning then removing the masks. And in practice it is not possible to do without masks, for the introduction of springs into a horizontally placed plate involves considerable risks of the springs catching in the windows.